UT Technique | Petrochemical
Plants | Reliability|
Dangerous and must be taken seriously
Steel suffers from hydrogen damage or high temperature hydrogen attack
when a seepage of hydrogen reacts with metal carbides
to form methane gas. This reaction decarburizes the steel, produces micro
cracks, and lowers the toughness of the steel, but does not necessarily
cause a loss of thickness. Detection of hydrogen attack is
important to ensure the safe operation of pressure vessels and piping
that is susceptible to such damage.
MC + 4H = M (M:metal) +CH4
The type of damage caused by hydrogen
attack depends on the source. The source of hydrogen in boiler
tubes is from a reaction of steam and steel. Hydrogen damage in
boiler tubes is, therefore, always associated with ID corrosion.
However, in chemical plant the source of the hydrogen is from the
flow stream (hydrocarbons) and, therefore, there is no
corrosion associated with hydrogen damage.
Damage caused by HTHA failure in a refinery
DEVELOPMENT OF NDE TECHNOLOGY BY A.S.
BIRRING SINCE 1981
Anmol Birring, President of NDE Associates, Inc., has been involved with the
development of NDT techniques for detection of HTHA since 1981. Most of the inspections
at that time used the ultrasonic amplitude technique, the reliability of which was
always questionable. Mr. Birring worked on several projects on this subject
with JGC, Japan, Idemitsu, Japan, EPRI and Chevron, Richmond. During that period, he
identified the relationship of ultrasonic backscatter with HTHA. In 1990, he was awarded US Patent,
4,890,496 for developing this technology. Since then backscatter
technique has been widely used worldwide in the petrochemical plants.
BACKSCATTER RECOGNITION TECHNIQUE (BRT)
Anmol Birring's inspection technique depends on recognizing the patterns of
backscatter signals in the time and spatial domains to HTHA. Detection of HTHA
is not straight forward and requires skill in identifying these unique
backscatter patterns that match with HTHA. This pattern recognition process
called BRT is
incorporated in our test process for quantitative assessment of HTHA.
We provide consulting services worldwide for HTHA assessment.
RELIABILITY OF HTHA INSPECTIONS
HTHA is dangerous, difficult to detect and can be missed. The reliability of HTHA inspections depends on the skill of the
inspector. We have run into cases where inspectors missed
last stages of HTHA
A case in
point is inspection of a C-0.5 Mo reactor that was tested by NDE Associates,
in 2004. The reactor at the Total refinery in Le Harve, France was placed in service in 1972
. The refinery was concerned of any damage by HTHA as the reactor was operating
above the C-steel curve. In 2004 NDE Associates inspected the reactor using the ultrasonic backscatter
technique and found significant levels of HTHA. Follow-up metallurgical,
replication and magnetic particle tests confirmed severe HTHA. Based on the
results, the refinery took the reactor
out of service. What is interesting is that an inspection done on the same
reactor in 2002 by another company using AUBT had
reported no HTHA. To achieve a high degree of reliablity and reducing chances to
plant owners must carefully
select inspectors to perform these inspections.
There are inspectors who have inspected over hundred components but never
reported HTHA. Do not take any chances.
attack (a) The dark area on the Boiler Tube ID represents hydrogen
damage. (b) Hydrogen Attack in the 18 mm thick pipe sample from a failure in a Refinery de-suphurization
plant in Japan. Failure date March 31, 1982. Note the
depth of HTHA at failure is 70 percent.
FOR Petrochemical Plants
Both the base metal and weld HAZ should
be inspected for hydrogen attack.
Base metal attack is detected by using a
combination of ultrasonic back-scatter and velocity measurements (4, 5,6,
7). Hydrogen attack increases the ultrasonic backscatter and reduces
the ultrasonic velocity in the material. In particular, HTHA increases the ratio
of S-wave to L-wave velocities. The backscatter and velocity ratio measurements
are applied to detect hydrogen attack. The ultrasonic backscatter technique was
developed by A. S. Birring in 1989 and was first applied at the Chevron Richmond
Refinery in 1989 (5). Details of the UT techniques for HTHA
inspection are given in references 4 and 5.
Application of the ultrasonic techniques for HTHA detection requires an individual
with a good understanding of the mechanism of HTHA and how it affects the
propagation and scattering of ultrasonic waves. One has to understand that
that while hydrogen attack affects velocity-ratio, backscatter and the frequency
of the reflected signal, other material anomalies can influence these
ultrasonic parameters as well and give a false calls. For example, ultrasonic
frequency of the reflected signal is affected by grain size, material microstructure and
surface roughness in addition to HTHA. Ultrasonic inspection for
this application is therefore not straight forward and requires a logical test
methodology and a skilled operator to identify specific backscatter patterns
related to detect HTHA. It is most important that owners diligently select
Application of ultrasonic backscatter for detection of HTHA.
Inspection is performed from the component OD. The
ultrasonic backscatter is ID connected.
Weld HAZ attack is detected using highly
focused phased arrays. Since cracking caused by
hydrogen attack is in the weld HAZ is extremely fine, the inspection is
done at a very high sensitivity.
Phased Arrays are not recommended for detection of base metal HTHA
Verification of attack can be done by
HTHA IN PETROCHEMICAL PLANTS
High temperature hydrogen attack (HTHA) in the
petrochemical is caused when the hydrogen from the stream
seeps into carbon and low alloy steels at high temperatures. Hydrogen reacts
with the carbides in the steel, decarburizing the steel and forming methane gas
bubbles at the grain boundaries with no loss of thickness. The methane gas bubbles
grow with time and result in micro cracking. The combination of micro cracking
and decaburization reduces the fracture toughness of steels and lead to major
HTHA is triggered in the components operating
at high temperatures and high hydrogen partial pressure. API
941's Nelson curves provide information about the safe operating
environment for components operating in hydrogen environments (1).
Components operating in the unsafe environments are susceptible
to hydrogen attack. The concern is especially true for C-1/2 Mo steels whose
curve has been been lowered and now dropped to the same level as that carbon
steel (1). Because of this drop, some of the C-1/2 Mo components originally
operating in the safe region are now in the unsafe region of the Nelson curves.
These components should either be inspected regularly for evidence of HTHA or
replaced with a higher grade of steel.
Hydrogen Attack can occur both in the
base metal and the weld HAZ. The attack in the base metal is wide
spread and distributed uniformly. In addition to temperature and hydrogen
partial pressure, the susceptibility of attack also also depends on the microstrocture ferrite/bainite or ferrite/pearlite with quasi M23C6 carbide (2). Weld attack is very localized
and grows along the HAZ. The susceptibility of attack in the weld
HAZ depends on the heat treatment. Post-weld heat treated welds
are less susceptible to hydrogen attack compared to the welds
that are not heat treated. There have been several cases where
the depth of the attack in the HAZ is greater than the depth
in the base metal. It is therefore imperative that both the base
metal and the weld HAZ be inspected for presence of hydrogen
Some of the factors that are important for
scheduling inspections are as follows:
- operating conditions (partial pressure of hydrogen and
temperature) relative to the operating limits provided in
API 941 (1).
- slow cooled C-1/2 Mo steels have less resistance to
hydrogen attack than normalized steels (2).
- post-weld heat treated welds are
less susceptible to hydrogen attack compared to the welds
that are not heat treated.
A general discussion of HTHA prediction is given in
PLANNING FOR HTHA
1. The preferred approach should be to replace C-1/2 Mo Equipment operating above the API 941 limits.
2. Reactors, heat exchangers shells and other thicker equipment must be
inspected at the earliest if immediate replacement is not possible.
Ultrasonic backscatter inspection must be performed by skilled inspectors
who who understand HTHA backscatter patterns and have a track record of detecting actual HTHA in the plant equipment, not
just inspecting for HTHA.
3. Ultrasonic calibration for backscatter and velocity ratio should be
performed/verified on a sample with service induced HTHA. This is very
important as it gives a representative test sample to establish and verify
4. Selection of inspection locations is critical. Inspection must be
performed at the hottest locations of the equipment. HTHA susceptibility is
highly dependent on the heat of the plate and is not very well understood.
Just a small difference in heat input or treatment can make a major difference in HTHA
susceptibility. So one plate/spool may be totally free of HTHA while the
adjoining plate/spool operating at the same temp/pressure conditions in the
same vessel may have high levels of HTHA. Each and every plate of the vessel
must therefore be tested at both ends. Preferred locations for HTHA are the
intersection points of the plates/spool.
5. Any suspect areas detected by ultrasonic testing must be verified by
replication of magnetic particle testing from the inside surface.
NDE Associates, Inc. has performed several HTHA inspections. Some of our clients are:
- Total Fina, Le Harve, France, 2004
- Total, Rome, Italy, 2005
- Total, Milford Haven, UK, 2007
- Chevron Oil, Richmond Refinery, CA. Conduct
follow up inspections after the April 1989 fire caused by Hydrogen Attack
- Chevron, Pascagoula, MS
- BP, Texas City, 2005
- other refineries in US, Canada
- Huntsman Chemicals, Melbourne, Australia.
- SK Oil, Korea. Conduct follow up inspections
after the failure caused by Hydrogen Attack. Failure date: May 13, 1999.
- CF (Fertilizers) Industries, Donaldsonville,
- Conoco Oil Company, Billings Refinery, MT
and Denver Refinery, CO.
- US Oil and Refining, Tacoma, WA
This website is solely for informational
purposes. In no event will this company be responsible for any loss or
damages resulting from any viewer's use of these materials.
- Steels for Hydrogen
Service at Elevated Temperatures and Pressure in Petroleum
Refineries and Petrochemical Plants, API Recommended Practice 941, American
- T. Ishiguro,
H.Yamamoto, K. Kawano, et al, "Metallurgical Effect on Hydrogen Attack Damage
Steels," Proceedings, 1996 ASME/ICPVT Pressure Vessels and piping Conference,
21-26 July, 1996
- K. Kawano, "Recent Activities in High Temperature
Hydrogen Attack," to be presented in 2005.
- A. S.
Birring, et al. "Method and Means for Detection of Hydrogen Attack by Ultrasonic
Wave Velocity Measurements" US Patent,
4,890,496, January 2, 1990
- A. S. Birring and K. Kawano,
"Ultrasonic Detection of Hydrogen Attack in Steels," Corrosion,
- A. S. Birring, M. Riethmuller,
and K. Kawano, "Ultrasonic Techniques for Detection of High Temperature
Hydrogen Attack," Materials Evaluation, February, 2005.
- C Leneve and M Richez, " Use of the the NDT for the Detection
of High Temperature Hydrogen Attack and Assessment of a Reactor", Esope 2010,
Paris, France, Sep 2010
inspections detect HTHA in a reactor.
HTHA inspection of
a reactor in France.
The reactor was taken out of service after inspections found HTHA. HTHA is
detected by recognizing unique backscatter patterns (BRT).